Siracusa et al. in U.S. Pat. No. 5,289,276, describe a video signal compression system which includes circuitry for compressing video data to generate a hierarchically layered encoded compressed signal, such as MPEG. MPEG is a coding standard developed by the Moving Picture Experts Group of the International Standardization Organization. To insure high quality signal recovery, for signals broadcast in a terrestrial medium, such as according to the NTSC broadcast standard, the MPEG compressed signal is packetized in transport cells, each of which includes signal identifiers and error detection codes. Portions of the MPEG data which are particularly important to signal reconstruction, at a corresponding receiver, are formed in redundant cells and interleaved at predetermined locations in the cell sequence. The predetermined locations are arranged such that if the original particularly important data is lost, the probability of the redundant cell being concurrently lost is extremely small. Thus, a receiver for reproducing the compressed data, will have the requisite information available.
The receiver in the Siracusa et al. system includes an inverse packet or cell processor which performs a plurality of functions including reconstructing the MPEG data stream from the cell payloads, storing the data in redundant cells for use in case of lost data, recognizing loss of data, and generating media error codes. The signal provided by the inverse cell processor is applied to an MPEG decompression apparatus. The decompression apparatus expects data to occur in a specific sequence defined by the MPEG protocol. If data is lost, the data sequence is corrupted. If the decompressor is not alerted to the corruption of the compressed data sequence it will incorrectly process subsequent data. In view of these possibilities the decompressor is arranged to recognize a particular codeword (designated herein as a media error code), in the compressed data stream. Responsive to the media error code, the decompressor is conditioned to ignore data until the next occurrence of a start code. On reception of the next start code the decompressor resynchronizes to the current data.
Depending on when and what data is lost, generation and insertion of the media error codes into the data stream has created significant system timing problems. The timing problems are completely obviated by the present invention.